The present invention relates to secure voice transmission.
The effort expended in searching for effective secure voice communication techniques has been considerable, especially in recent years. For example, many analog secure voice techniques, or speech scramblers, have been proposed and widely discussed. See, for example, N. S. Jayant et al, "A Comparison of Four Methods for Analog Speech Privacy", IEEE Trans. Comm., Vol. COM-29, No. 1, January 1981, and references cited therein. There is, however, a general consensus that digital encryption techniques, such as described in W. Diffie and M. E. Hellman, "Privacy and Authentication: An Introduction to Cryptography", Proceedings IEEE, Vol. 67, pp. 397-427, March 1979, are more effective from the cryptanalytical point of view. That is, they provide much greater security from either casual or intentional eavesdropping. A fundamental drawback of digital encryption, however, is that toll quality transmission of encrypted speech cannot be achieved at the data rates afforded by current voice band data technology. At best, only "adequate" speech quality can be achieved.